South Korea has conducted drills twice this year (Wikimedia Commons)

The live-fire drills took place from five islands, one of which - Yeonpyeong - was attacked by the North in 2010, killing four. 

The New York Times reported that many of the 8,700 civilians on the five islands fled to bomb shelters after the North warned that it might retaliate.

South Korea notified the North of the drills on Sunday, and Pyongyang threatened retaliation Sunday and again early Monday.

The drills lasted two hours.

From BBC:

"The aim of the exercise was 'to test our weapons at Marine Corps units on the Yellow Sea and also to maintain our military's combat readiness in the area,' an official with the Joint Chiefs of Staff (JCS) was quoted by Yonhap as saying."

This was the second time this year the South conducted drills.